BACKGROUND: Infection by dermatophytes affects 40% of the world population and represents 30% of all mycotic cutaneous infections, onychomycosis being the most frequent disease of the nails, counting for 18 to 40% of all onychopathies. Onychomycosis is caused primarily by dermatophytes, Candida spp. and other non-dermatophyte fungi. Among the etiologic agents the most common are dermatophytes and especially Trichophyton rubrum. Nowadays Candida spp. can invade the distal and proximal nails. Onychomycosis due to non-dermatophyte fungi has been increasing in prevalence and in view of the clinical similarity with onychomycosis caused by dermatophytes, a laboratorial diagnosis is necessary for its differentiation. OBJETIVES: The objective of the present work is to evaluarte the occurrence of onychomycosisin the city of Rio de Janeiro. METHODS: Out of 2920 patients attended between January 1998 and December 1999, 1416 presented abnormalities in their nails. RESULTS: Mycological confirmation of onychomycosis was performed in 565 patients and the estimated prevalence in the city of Rio de Janeiro is 19.34%. Women were more affected than men, with a ratio of almost 2:1. The distribution according to etiological agent of 224 patients with onychomycosis and who presented positive culture was: dermatophytes 64.7%; Candida spp. 30.1%; and non-dermatophytes 5.2%. CONCLUSION: The fungal flora changes completely periodically in the world in terms of its quantitative and qualitative composition as it is affected by several environmental factors. Consequently, a periodic exam of the composition of the microflora can be interesting and of epidemiological and therapeutic importance.